Perfluoroalkyl iodides corresponding to the empirical formula C.sub.n F.sub.2n+1 I where n denotes the number of carbon atoms (or degree of carbon condensation) are used as synthesis intermediates for numerous applications relating in general to the field of fluorinated surface-active substances and more particularly the bases for extinguisher formulations, hydrophobic and oleophobic finishes for the treatment of textiles or of paper, and more recently for applications of a medical nature (contrast agents or oxygen carriers).
Perfluoroalkyl iodides are obtained by telomerization of tetrafluoroethylene (taxogen) by pentafluoroethyl iodide C.sub.2 F.sub.5 I (telogen), which itself is prepared by the action of iodine and iodine pentafluoride on tetrafluoroethylene in the presence of a catalyst. These two reactions may be coupled as described in French Patent FR 1 385 682, but in the majority of cases C.sub.2 F.sub.5 I is first prepared and is then used in the telomerization which leads to straight-chain perfluoroalkyl iodides. In order to gain access to branched perfluoroalkyl iodides, a secondary perfluoroalkyl iodide, such as heptafluoroisopropyl iodide CF.sub.3 CFICF.sub.3, is used as telogen.
The telomerization reaction may be carried out in accordance with at least three methods, the essential difference between which lies in the mode of activation, which may be:
either free radical, with the aid of diverse peroxide initiators, such as in the processes which are the subject of French Patents FR 2 035 913, FR 2 325 665 and U.S. Pat. No. 3,226,449,
or catalytic, by the involvement of a redox system as in the processes according to French Patents FR 2 028 781 and FR 2 098 335,
or, finally, thermal, as in the processes which are the subject of French Patent FR 1 415 498 and U.S. Pat. No. 3,404,189.
In all of these processes a more or less wide distribution of different chain lengths is obtained and, even in the processes using catalytic initiation which are said to be more selective, it is difficult to attain relatively narrow distributions for telomers of class i ranging from 2 to 5, i denoting the number of molecules of tetrafluoroethylene telomerized by the telogen.
When these perfluoroalkyl iodides are used as synthesis intermediates, the heavier telomers (those of class i&gt;5) are less soluble in the conventional reaction mixtures and have reaction kinetics distinctly slower than those of the more lightweight telomers (1.ltoreq.i.ltoreq.5). This results in an adverse accumulation of unconverted intermediates throughout the chain of reactions leading to the final product, with a direct influence on the quality of the latter, in particular in the applications making use of its physicochemical properties, such as, for example, the surfactant activity.
British Patent 1 314 668 describes a process for separating off telomers having a degree of carbon condensation of less than 14 by extraction with the aid of a solvent. This method gives rise to a loss of products and is therefore not satisfactory from the economic standpoint.
One process for reducing the proportion of heavy telomers (&gt;C12) consists in increasing the telogen/taxogen ratio (see French Patents FR 2 028 781 and FR 2 035 913), but this implies a low degree of conversion of the telogen with high recycle ratios and leads to a distribution selectively oriented towards the telomer of class 1. Thus, according to French Patent FR 2 035 913, the telomerisation of C.sub.2 F.sub.4 by C.sub.2 F.sub.5 I in a C.sub.2 F.sub.5 I/C.sub.2 F.sub.4 ratio of 2.9 leads to the following distribution:
______________________________________ CLASS TELOMER % ______________________________________ 1 C.sub.4 F.sub.9 I 42.2 2 C.sub.6 F.sub.13 I 23.8 3 C.sub.8 F.sub.17 I 14.6 4 C.sub.10 F.sub.21 I 8.64 5 C.sub.12 F.sub.25 I 6.25 .gtoreq.6 .gtoreq.C.sub.14 4.52 ______________________________________
It is also possible to modify the reaction time and the degree of conversion of the taxogen. In general, low degrees of conversion improve the selectivity for the class 1 telomer. In practice, the multiplicity of steps which would be necessary to obtain a class i telomer in a selective manner leads to a complex technology and to a result which is equivalent in respect of the proportion of heavy products. In the processes which operate continuously, the optimization for a class i telomer is achieved by complete or partial recycling of the telomers of class i-k where 1.ltoreq.k.ltoreq.i-1.
Now, numerous applications make use of telomers belonging to the C.sub.6 -C.sub.12, and more particularly C.sub.8 -C.sub.10, cut. The above processes do not permit optimization of the production of RfI telomers for this cut without inevitably increasing the concentration of heavy products.